Semiconductor wafers are subject to a variety of processing steps in the course of the manufacture of semiconductor devices. The processing steps are usually carried out in a sealed vacuum chamber of a wafer processing system. One such group of processing steps is referred to collectively as chemical vapor deposition or CVD. CVD encompasses deposition of material layers onto a wafer by a reaction of various chemicals which are usually in a gaseous or vapor state. However, CVD processes might also involve chemical etching of the wafer. Various of the chemical gases and vapors used in CVD process are corrosive to some of the metal parts and devices utilized in the reaction chamber of a wafer processing system. Accordingly, it is necessary to isolate and protect such parts and devices from the chemical gases.
CVD processes generally involve heating the wafer prior to deposition or etching. CVD is typically performed in a cold wall reactor, where the wafers to be coated or etched are heated to a reaction temperature on a wafer-supporting susceptor while other surfaces of the reactor are maintained at sub-reaction temperatures to prevent the deposition of films thereon. For tungsten chemical vapor deposition, for example, the wafer is heated while the reactor walls are cooled, often to around ambient room temperature. Alternatively, for titanium nitride chemical vapor deposition, the walls may be heated above ambient room temperature, but to a temperature below that of the wafer being coated.
Heating substrates in a chemical vapor deposition environment within a reactor is often difficult because the heating elements used to heat the wafer must be isolated from the corrosive chemical vapors and gases utilized in the CVD process. One possible solution involves the use of infrared (IR) radiation to heat the wafer which is directed onto the wafer from outside the reaction chamber through IR windows formed in the reactor. However, temperature uniformity is often difficult to achieve with IR radiation, because the IR windows become coated with material layers by the CVD process leading to inconsistent heating of the wafer. Alternatively, resistive or resistance-type heaters coupled to the wafer-supporting susceptor within the reactor have provided better temperature uniformity and stability.
Generally, resistance-type heaters are mounted within a wafer-supporting susceptor structure to a susceptor backplane or platen and operate to heat the susceptor backplane and simultaneously heat the wafer to the desired operating temperature. However, since resistance-type heating elements must heat the susceptor backplane while heating the wafer, the thermal response time may be slower than is desired to achieve maximum throughput of the wafers. Therefore, to improve the thermal response time, a gaseous medium, such as helium, is pumped between the susceptor backplane surface and the wafer supported on the backplane which enhances the transfer of heat between the susceptor backplane and wafer.
A typical semiconductor wafer-supporting susceptor provided within a reaction chamber has fixed to its bottom a susceptor drive support frame. Rotatably mounted within the drive support frame is a hollow susceptor drive shaft which is utilized to rotate the susceptor, when desired, during CVD processing. The hollow susceptor drive shaft is rigidly connected to the bottom of the susceptor. A hollow space within the drive shaft communicates with the interior of the susceptor inside the reaction chamber. Penetrations and openings are made in the susceptor and susceptor backplane so that a vacuum hold of the wafer may be accomplished. Ordinarily, the vacuum pressure inside the hollow drive shaft and within the susceptor interior is maintained at a pressure sufficiently lower than that of the reaction chamber to develop a vacuum within the susceptor which operates as a vacuum chuck to hold a wafer against the heated susceptor backplane during processing. However, the penetrations in the susceptor and a susceptor backplane, as well as the interface between the drive shaft and susceptor, provide paths for corrosive chemicals to enter the interior of the susceptor and come into contact with the heating elements. As a result, the heating elements may be damaged, especially whenever a wafer is not held to the susceptor backplane under vacuum to block the backplane penetrations and openings.
Furthermore, penetrations through the susceptor and backplane must also be made to provide a passage for the heating gas to reach the backside of the wafer. Again, the susceptor heating elements are somewhat isolated from the corrosive chemical vapors as long as the penetrations are covered by a wafer, but when the wafer is not present, there is a direct path for corrosive gases from the reaction chamber to the heating elements through the heating gas penetrations.
Accordingly, there is a need for a susceptor or similar wafer-supporting device for use in a CVD environment which provides adequate isolation and protection of the heating elements from the corrosive chemical vapors both during processing of a wafer and upon removal of the wafer. There is further a need for such a device which isolates the heating elements while providing satisfactory heating of the wafer as well as supplying backside heating gas to the wafer for more efficient heat transfer.